CDKL5 and infantile spasms: Mutations in the X-linked Cyclin-Dependent Kinase-Like 5 gene, CDKL5 (Mendelian Inheritance in Man, MIM: 300203; previously known as STK9), cause a range of phenotypes, including EIEE2 (MIM: 300672), a form of early infantile epileptic encephalopathy [1, 2], and infantile spasms [3–7].